The use of ultrasound Doppler for Spectral measurement of blood flow velocity in arteries and veins is well established. One widely used procedures for making such measurements is based on three typical stages: an initial identification of the target area (where flow is to be measured) using ultrasound imaging; placement of a marker on the appropriate position on the image; and switching the echo device from Imaging mode to Spectral Doppler Examination mode in order to display the flow velocities in real-time. This procedure can be used, for example, to measure the blood flow in a pulmonary vein.
Another procedure, which is relatively new, is used for Trans Cranial Doppler (TCD) measurements, as well as some peripheral vascular studies. In this procedure the ultrasound beam is directly aimed at the known location of the target, without relying on imaging. As the structure and positioning of the human skull and its constituents are relatively fixed and known, specific vessels such as the arteries of the circle of Willis, at the base of the brain, are being studied in this procedure by echo Doppler alone (i.e. without imaging). The fact that the flow velocity measurements can be made without imaging enables one to do the measurements through the bones of the skull that attenuate and scatter the ultrasound beam to such an extent that practical images cannot be obtained.
While trans-cranial Doppler measurements are now in routine use to study structures in the brain, applying this technology trans-thoracically monitor pulmonary vessels was heretofore considered impossible. This is due to the fact that the lungs contain numerous air pockets that attenuate and scatter ultrasound far more than bone. In view of this, except for the initial, large, segments of the pulmonary vessels that are not masked by lung tissue, arterial and venous flow velocity in the pulmonary vasculature and the lung tissue itself have not been studied by Doppler ultrasound.